1. Field of the Invention
The present invention relates to a photoelectric converting device.
2. Related Background Art
In the conventional art of the photoelectric converting devices, there is for example known a device of a bipolar transistor type.
FIG. 9 shows, in a schematic cross-sectional view, an example of the conventional bipolar photoelectric converting device, in which there are provided a silicon substrate 1 of n.sup.- or n.sup.+ -type by doping of an impurity such as phosphor (P), antimony (Sb) or arsenic (As); an n.sup.- -area 2 formed for example by an epitaxial process; a p-area 3 for accumulating photogenerated carriers; an n.sup.+ -area 5 constituting an emitter for obtaining an output signal; an n.sup.+ -area 7 constituting a channel stop or connected with the collector; an insulating film 100 for isolating the element; and an electrode 200 composed for example of polysilicon or metal.
In such a photoelectric converting device, various developmental efforts have been made to achieve a finer geometry and a higher level of integration, in order to respond to the requirement for compactization of the solid-state image sensor or the like employing such photoelectric converting device.
However, such conventional photoelectric converting device, if made with a finer geometry, results in a reduced amount of photogenerated charge due to the diminished aperture rate, leading to a lowered photosensitivity.
In the conventional bipolar photoelectric converting device, the photosensitivity is represented, in approximate manner, by the following equation: ##EQU1## wherein i.sub.p is the photoinduced current density per unit area; A.sub.s is the area of aperture; t.sub.s is the accumulation time, and C.sub.bc is the base-collector capacitance.
As will be apparent from the equation (1), the sensitivity Se decreases with the reduction of the aperture area A.sub.s.